Locking exposure on the 7D

edit: When I read PP 269 in the manual, which I referenced for you. It said 'linkable to any AF point'. I may have had to do a link operation to achieve what I and TTMartin have told you about. I can't remember. But I know that my camera is linked, so that the in evaluative mode, the strong exposure weighting follows the selected AF point. Try and see if yours is linked. I can't remember doing it, but I may have had to link them. This much I am positive of, the camera can do it!!

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kind regards
Dale